Slurries are processed in many industries ranging from food processing to biofuel production. In the production of biofuels, for example, the addition of water or other liquid to the incoming feedstock to form a slurry facilitates the transportation and mechanical handling of the feedstock.
There has been impetus in many industries in recent years to reduce the liquid content of slurries to improve process economics. Processing of slurries with low liquid content can considerably reduce energy and chemical usage in a plant. For example, in ethanol production, feedstock slurries can be deliquified prior to energy intensive operations such as heating of the feedstock slurry, which is employed in a cooking operation conducted with acid or alkali, known as pretreatment. The low liquid content in the incoming slurry requires less steam to heat during pretreatment, thus reducing energy costs. Moreover, lower volumes of liquid in the slurry result in reductions in equipment size, which reduces capital and operating costs.
Various devices are known for mechanically removing liquid from a slurry to increase its solids content. One such device is a press that operates by pressing liquid out of the slurry by the mechanical action of a conveyance member, which is a device that advances material through the press. An example is a screw press that removes liquid by an axially mounted rotating screw. The screw rotates within a shell containing passages through which liquid can pass, but that retains most of the solids. After initial liquid removal, the resultant partially deliquified slurry is further compacted by advancing it through a plug formation section of the screw press. This compacting action forms an integrated mass of material, known as a “plug”. The plug formation section may be a tube attached to the discharge end of the deliquifying section. Plug formation in a screw press increases the internal pressure of the slurry leading to increased removal of liquid and may also form an internal pressure seal.
It is important in many industrial applications that the material discharged from the press has a consistency that does not vary considerably from a target range. If material discharged from a press is too thick, wear on the press increases quickly as well as the load on the motor. Increases in discharge consistency may also lead to plugging of the press or downstream equipment. Conversely, material that is too thin may increase chemical and steam usage. Although the material discharged from the press can be evaporated or dilution liquid can be added to the discharged material to reach a desired consistency, these additional operations add cost to the process.
One means for controlling discharge consistency from the press is through the use of flow restriction devices, also known in the art as “restrictors”, placed at the discharge end of the press. Flow restriction devices partially obstruct the flow of the plug from the discharge end of the press. By obstructing the flow, counter pressure is built up against the plug, and the degree of compaction of the slurry is increased, which in turn leads to increased liquid removal from the slurry in the deliquifying section of the press.
Fine control of discharge consistency from a press can be achieved by specially designed mechanical restriction devices. Such devices are known in the art as “variable discharge restrictors”. Examples of discharge restrictors include those that function by biasing a door against the plug at the discharge section of the press. When the back pressure of the door is overcome by the force of the plug, the door slides rearwardly, allowing for ejection of plug material. The back pressure on the door can be controlled by hydraulic pressure or by springs, which in turn determines the discharge consistency of the plug. Other mechanical devices that are used to adjust the degree to which the discharge flow of material from the press is impeded include spring-loaded flippers, which rotate with the conveyance member and restrict the discharge (see U.S. Pat. Nos. 3,256,808 and 3,135,193). Variations of these concepts have been implemented on presses over the years.
Although these mechanical designs can control the discharge consistency from the press within a set range, they add mechanical complexity. This increases capital costs and requires increased maintenance. Thus, there is a need in the art for a more simple and cost effective means for controlling the discharge consistency of a press. Such a process would desirably improve process reliability, without the disadvantages of increased mechanical complexity.